The invention relates generally to catalyst systems and more particularly to catalyst configurations for catalyst systems which operate at high temperatures.
Catalyst systems are designed to operate within a prescribed operating temperature range. If the prescribed temperature range is exceeded, the catalyst activity may be destroyed or otherwise rendered ineffective. In particular, if a low ignition temperature is to be maintained at the front end or upstream portion of the catalyst, the catalytic activity at the front end is to be maintained at a substantially constant level.
In most present catalyst systems which operate essentially adiabatically, the normal operating temperature of the downstream portion of the system (the hottest part of the system) may be as high as about 815.degree. C. or slightly higher, and catalyst compositions are available which provide both satisfactorily high activity and temperature stability at this temperature. Only the downstream portion approaches the highest temperatures because, during the steady state operation of an essentially adiabatic catalyst system supporting a typical exothermic reaction, a temperature distribution is established along the length of the catalyst, the downstream portion of the catalyst being at the highest temperature and the initial or upstream portion of the catalyst being at a lower temperature.
In combustion systems utilizing a catalyst, for example, of the kind disclosed in copending application Ser. No. 358,411, filed May 8, 1973, and entitled "Catalytically Supported Thermal Combustion", operating temperatures on the order of about 950.degree.-1750.degree. C. are not uncommon at the downstream portion of the catalyst. Such systems therefore utilize a catalyst composition which retains substantial amounts of catalyst activity at high temperatures. The catalyst is to maintain a low ignition temperature to be effective. However, some catalyst compositions may be relatively less active when compared to more highly active catalysts which are used at lower temperatures.
Honeycomb catalyst systems such as that described in copending application Ser. No. 358,411, may be operated so that the temperature of the upstream portion of the catalyst configuration is determined primarily by heat transfer by both thermal conduction and radiation, from the downstream portion of the catalyst. In honeycomb catalysts in particular, heat transfer due to radiation may be substantial because of line of sight paths from the downstream portion to the upstream portion. The temperature of the upstream portion under steady state conditions can be accurately estimated in accordance with the accepted principles of heat transfer by taking into account (1) the rate of heat transfer due to thermal conductivity from the downstream portion to the upstream or initial portion of the catalyst system and (2) the rate of heat transfer due to radiant heat transfer from the downstream portion of the catalyst system to the upstream portion. The rate of heat transfer due to thermal conduction is proportional to the temperature difference between the upstream and downstream portions, while the rate of heat transfer due to radiation is proportional to the difference between the downstream temperature raised to the fourth power and the upstream temperature raised to the fourth power. Thus, when the downstream temperature is very high, the temperature at the initial portion is determined primarily by radiant heat transfer, and as a result, the temperature of the initial portion of the catalyst system is higher than would be predicted by thermal conduction alone.
The very high temperatures at the downstream end of a honeycomb catalyst may be important and critical because the corresponding higher temperatures at the initial portion may restrict the use of highly active catalyst compositions at the initial portion of the catalyst system. This may be a serious problem if an active catalyst is used to advantageously provide the system with a relatively low ignition temperature. Thus, in order to maintain a low ignition temperature, catalyst activity is to be maintained. However, high temperatures at the initial portion maintained under steady state conditions may tend to deactivate the catalyst composition at the initial portion of the catalyst system, thereby causing an undesirable rise in the ignition temperature of the system for subsequent start-up.
One deactivation mechanism is a loss of base surface area, for example due to sintering of the base composition. The surface area of the base can be measured by the well known method developed by Brunauer, Emmett and Teller. Another deactivation mechanism would be the growth of metal crystallites and the corresponding loss of active metal surface area. The size of the crystallites can be measured by chemisorption, for example by measuring the amount of H.sub.2 or CO which is adsorbed under specified test conditions. The above test methods can also be used to provide measurements to correlate with the catalytic activity of the catalyst.